Wet oxidation with the aid of a porous catalytic contactor

ABSTRACT

A process for oxidation of oxidizable materials which are dissolved or suspended in a liquid phase. A contactor in the form of a porous membrane is used. The contactor is designed such that an oxidizing phase flows along one surface thereof while the phase to be oxidized flows along another surface. The oxidation is catalyzed by (i) a catalyst material which constitutes the porous membrane or is deposited in or onto a porous membrane support, or (ii) a catalyst material which is supplied to one or both of the feed streams comprising the oxidizing phase and the phase to be oxidized.

This application claims the benefit of international application numberPCT/NO02/00100 filed Mar. 11, 2002, which claims priority of Norwegianpatent application 20011238 filed Mar. 12, 2001. The internationalapplication was published under PCT Article 21(2) in the Englishlanguage.

The present invention relates to a liquid treatment by catalyticoxidation.

To achieve oxidation of oxidizable, dissolved or suspended substances orparticulate material in a liquid phase, the oxidation process of thepresent invention utilizes a catalytic contact element, hereafter calleda contactor.

The primary application of the present invention is to overcome problemsassociated with the treatment of industrial waste water and tofacilitate the re-use of water, minerals and other raw materials inindustrial processes. In addition, the invention's process would beuseful in breaking down toxic substances and substances that are notbiodegradable. The invention's process makes it possible to achieveoxidation in the liquid phase at a lower pressure and temperature thanwhat is feasible with today's processes for wet oxidation andcombustion. Also, the process has lower space requirements than dotoday's processes for biological treatment of waste. The low temperatureleads to fewer corrosion problems in the processing equipment than withtoday's equipment for wet oxidation, enabling the use of less expensivematerials. The result is that the process of the present invention isless demanding with regard to energy and costs, both for investment andoperation, than the current technology. Furthermore, the presentinvention's process has a wide range of applications.

The invention's process would also be useful for purposes other thanwaste treatment, for example, for controlled oxidation in themanufacturing of chemicals and products.

The oxidizable suspended or dissolved material in the liquid mayoriginate from any source whatsoever. For example, it may be waste waterfrom industry, agriculture or a similar activity, especially, of course,in those situations where a discharge of this type of oxidizablematerial would constitute a strain on, or contamination of, the exteriorenvironment.

In given situations, such a process also permits a recovery of theoxidizable material after the oxidation, and it enables a far greaterdegree of recycling of purified waste water instead of discharge.

The present invention makes use of a technology that utilizes a porous,catalytic contactor. This porous, catalytic contactor consistsessentially of a porous membrane support that is loaded with aheterogeneous catalyst.

The porous contactor may consist of one or several layers made ofoxides, polymers or any other materials, and may have any convenient andexpedient shape whatsoever, for example, a tube, a hollow fiber, amultichannel tube, or a plate, or it may have any other practical form.

As the catalyst there may be used a precious metal, a non-preciousmetal, a metal oxide or any other material. It is also possible that thecontactor-will not be loaded with a separate catalytic material, butthat the porous membrane material in itself will have a catalyzingeffect for the oxidation reaction.

In the process according to the invention, one or both of the feedstreams can contain a homogeneous catalyst or one or both of the feedstreams can contain a heterogeneous catalyst.

Further the catalyst may be supplied batchwise together with either ofthe two feed streams or the catalyst may be supplied batchwise duringintervals while one or both of the two feed streams are temporarilystopped.

The oxidizing phase is a fluid, thus it is a gas or a liquid containingan oxidizing agent that may be air, oxygen, oxygen-enriched air, ozone,hydrogen peroxide, or another oxidizing agent.

The resulting process according to the invention can be carried out inaccordance with various process modes, for example, in batches, byrecycling, by continuous feeding and bleeding, or by a continuousflow-through method.

In accordance with this the present invention relates to a process forthe oxidation of oxidizable materials which are dissolved or suspendedin a liquid phase, and the invention is characterized in that there isused a contactor in the form of a porous membrane, said contactor beingdesigned such that an oxidizing phase flows along one surface of thecontactor while the phase to be oxidized flows along the other, and inthat the oxidation is catalysed by (i) a catalyst material whichconstitutes the porous membrane or is deposited in or onto a porousmembrane support, or catalysed by (ii) a catalyst material which issupplied to one or both of the feed streams comprising the oxidizingphase and the phase to the oxidized, respectively, which flow along saidporous membrane support.

In one embodiment there is as contactor used a porous, ceramic supporthaving a platinum-containing top layer as catalyst.

In a further embodiment there is as porous membrane catalyst supportused an oxide, another inorganic material, an organic polymer materialor any material.

The catalyst may be a precious metal, a non-precious metal, an oxide orany other: material.

The invention shall be described below in more detail with reference tothe attached figures, wherein:

FIG. 1 shows a flow chart for a general apparatus that utilizes theinvention's process;

FIGS. 2 and 3 show two examples of the way the catalytic contactor canbe arranged in the reactor in order to utilize the invention's process;

FIG. 4 shows the principle for the oxidation in the catalytic contactoraccording to the invention;

FIG. 5 shows a drawing based on a microphotograph in cross section andan embodiment form of a contactor with a catalyst according to theinvention; and

FIG. 6 shows curves illustrating test results obtained through the useof the invention's process.

In FIG. 1 is shown a flow chart of an apparatus that utilizes theinvention's process. The liquid or suspension to be oxidized enters thepump 2 at 1. The flow rate is regulated by a valve 3, which is coupledto a flow regulator, and the liquid is conducted into the reactor 4 onone side of the catalytic contactor 5. The liquid is conducted on theone side of the contactor, along the contactor's porous surface, and ispassed out of the reactor at 6. The oxidizing agent, which may be air,oxygen, enriched air, hydrogen peroxide or another oxidizing agent, isintroduced at 7 to the pump or the compressor 8. The flow rate of theoxidizing agent is regulated by a valve 9, which is coupled to a flowregulator, and the liquid is conducted into the reactor 4 on the otherside of the catalytic contactor 5. The oxidizing agent is conducted onthe other side of the contactor, along the contactor's porous surface,and is passed out of the reactor at 10. In the illustrated case, the twostreams are conducted co-currently on each side of the porous catalyticcontactor, but it is also conceivable that the two streams can beconducted counter-currently or cross-currently on each side of thecontactor. The pressure conditions in the reactor 4 and the pressuredifference between the two sides of the contactor 5 are regulated bymeans of valves 11 coupled to pressure regulators at the outlets fromthe reactor. The temperature in the reactor is constantly maintained atthe desired level with the aid of a heating/cooling system 12, which mayalso provide for the recovery of heat. In those cases where theoxidizing agent is a gas, the outlet from this side of the contactor maybe connected to a separator 13, such that any liquid in this outlet canbe fed into the oxidized liquid stream 14.

In FIGS. 2 and 3 are shown two alternative embodiments of the reactor.FIG. 2 shows a reactor composed of a plurality of tubular contactors 20,whereas FIG. 3 shows parts of a reactor assembled from a plurality ofplate-shaped contactors 30. The oxidizable liquid is fed in at 21, andthe oxidizing agent is introduced at 22. The oxidized liquid is passedout at 23, whereas unused oxidizing agent and portions of the oxidationproducts are removed at 24. Alternatively the streams can be exchangedsuch that the oxidizable liquid is introduced at 22 and the oxidizingagent is introduced at 21. The oxidized liquid is then brought out at24, whereas unused oxidizing agent and parts of the oxidation productsare passed out at 23.

FIG. 4 illustrates the principle for the invention's mode of operation.In the porous contactor 40 there is a tone with a material 41 whichfunctions as a catalyst for the oxidation. The oxidizable material 42,which may be, for example, organic molecules dissolved in water, isconducted toward the one side of the porous contactor. The oxidizingagent 43 is conducted in toward the other side of the contactor. Theoxidizable material diffuses into the pores of the contactor, where itmeets the oxidizing agent on, or in the proximity of, the catalyst 41,which causes a spontaneous oxidation of the oxidizable material to takeplace. The reaction products 44 and 45 from the oxidation, which may bein gas or liquid form, diffuse out of the contactor on one or both ofthe element's two sides. The reaction products may be completelyoxidized substances, for example, water, carbonates, carbon dioxide ornitrogen oxides, or partially oxidized substances, for example,carboxylic acids.

FIG. 5 shows a drawing based on a microphotograph of a contactor. In theillustrated case, the support layer 50 has an average pore diameter inthe range 5 to 10 μm to obtain very weak hydraulic resistance and athickness of a few mm.

Onto this support, is laid an intermediary layer 51 in themicrofiltration range. The thickness is around 20 μm.

Onto this intermediate layer, is laid an additional intermediate layer52 in the ultrafiltration range with a thickness around 5 μm. Thisadditional intermediate layer contains some of the catalyst, in thiscase platinum. The support layer and the first intermediate layer donot, in this case, contain catalyst.

Finally, the contactor comprises a top layer 53 with a thickness around1 μm. This layer contains platinum as catalyst.

FIG. 6 shows the results that were obtained through the use of acontactor with catalyst as shown in FIG. 5. The contactor in this casewas shaped as a tube having a length of 100 mm, an outer diameter of 10mm, and an inner diameter of 6 mm. FIG. 5 shows the cross section of anarea near the inner surface of a similar tube. The reactor was utilizedwith a model solution consisting of 5 g/l of formic acid in water. Areservoir that contained between 1 and 2 liters of test solution wasused for each test, and the solution was pumped from the reservoir,through the reactor, and back to the, reservoir in a recycling loop.Through the reactor the solution was conducted along the outside of thecontactor tube. The fluid velocity was 50 ml/min. At the same time,compressed air was conducted through the tube at a velocity of between50 and 100 mil/min. Samples of the solution were taken from the outletof the reactor at irregular intervals. Standard methods for chemicalanalysis were used to determine the content of oxidizable material(chemical oxygen demand, COD) or the content of organic bound carbon(total organic carbon, TOC) in these samples. The results from theseanalyses were converted to degree of oxidation by dividing the values bycorrespondingly calculated values for the model solution prior to thetests. The degree of oxidation constitutes the Y-axis in FIG. 6. TheX-axis represents the time from start of the test to the time when thesample was taken, divided by the total volume of model solution in theapparatus at the specified test. In one of the tests, which yielded theresults indicated by the points marked 60 in FIG. 6, the pressure in theapparatus was 1 bar and the temperature was 80° C. In another test,which yielded the results indicated by the points marked 61 in FIG. 6,the pressure was 10 bars and the temperature was 150° C. In both of thetests, the pressure difference between the two sides of the contactorwas less than 0.2 bar.

The results of the tests that are shown in FIG. 6 indicate that it ispossible to achieve over 50% oxidation of formic acid under very mildprocessing conditions. Even at a pressure of 1 bar and only 80° C. it ispossible to achieve nearly 50% oxidation. In connection with the testsmentioned above, a test was also conducted with a contactor that was notimpregnated with catalyst. This test was carried out at a pressure of 10bars and a temperature of 150° C., and the results showed that nomeasurable oxidation took place in this case.

1. A process for wet oxidation of oxidizable materials which aredissolved or suspended in a liquid phase, comprising: providing acontactor in the form of a porous membrane, said contactor beingdesigned such that an oxidizing phase containing an oxidizing agentflows along one surface of the contactor in a first feed stream whichdiffuses into the porous membrane and the liquid phase to be wetoxidized flows along the other surface of the contactor in a second feedstream which diffuses into the porous membrane, and catalyzing the wetoxidation of the oxidizable material in said liquid phase by one of (i)a catalyst material which constitutes the porous membrane or isdeposited in or onto a porous membrane support, and (ii) a catalystmaterial which is supplied to at least one of the first and second feedstreams, wherein the catalyst material remains unchanged by saidoxidation; and said oxidation occurs within said porous membrane.
 2. Theprocess according to claim 1, wherein the contactor comprises a porous,ceramic support having a platinum-containing top layer as catalyst. 3.The process according to claim 2, wherein the oxidizing a agentcomprises one of air, oxygen-enriched air, oxygen, ozone, and hydrogenperoxide.
 4. The process according to claim 1, wherein the oxidizingagent comprises one of air, oxygen-enriched air, oxygen, ozone, andhydrogen peroxide.
 5. The process according to claim 1, wherein thecontactor has a shape of one of a tube, a hollow fiber, a multichanneltube, and a plate.
 6. The process according to claim 1, wherein theporous membrane support comprises one of an oxide, an inorganicmaterial, and an organic polymer material.
 7. The process according toclaim 1, wherein the catalyst material comprises one of a preciousmetal, a non-precious metal, and an oxide.
 8. The process according toclaim 1, wherein one or both of the feed streams contain a heterogeneouscatalyst.
 9. The process according to claim 1, wherein one or both ofthe feed streams contain a homogeneous catalyst.
 10. The processaccording to claim 1, wherein the catalyst material is suppliedbatchwise together with either of the two feed streams.
 11. The processaccording to claim 1, wherein the catalyst material is suppliedbatchwise during intervals while at least one of the feed streams istemporarily stopped.